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The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO 2) in the storage formation at depth. One of the critical elements of the safekeeping of CO 2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO 2 through a fault damage zone, (ii) the interaction of the CO 2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO 2 -saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO 2 storage at depth. The mobility of the CO 2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone. [ABSTRACT FROM AUTHOR]

Experimental reconstruction of temperature - altitude in the past remain often elusive. With the development of a combined vacuum crushing and sieving system (CVCS), it is now possible to determine paleotemperatures from the noble gases dissolved in minute amounts of water from inclusions in speleothems that have grown under a broad range of climatic conditions. Here we present noble gas temperature (NGT) estimates during the last deglaciation, which are based on two stalagmites from Milandre Cave (stalagmite M6) and Grotte aux Fées de Vallorbe (stalagmite GEF1), Jura Mountains, Switzerland. The caves are located at different geographical altitudes (Milandre Cave: 373 m a.s.l. and Grotte aux Fées de Vallorbe: 895 m a.s.l.) and thus allow for a reconstruction of the respective temperature – altitude gradients within Switzerland during the last deglaciation. Our reconstruction shows that the past temperature – altitude gradients are within 1 σ error in agreement with the modern temperature – altitude gradient, suggesting that the local temperature – altitude gradient was rather stable over time. In addition to the noble gas analysis, we complemented our study with deuterium (δ D FI) and oxygen isotope (δ 18 O FI) measurements of fluid inclusion water. In combination with NGTs, this allows us to reconstruct the past δ 18 O FI – δ D FI / temperature relation. These reconstructions show that the temporal temperature sensitivity of δ 18 O FI and δ D FI (Δ(δ 18 O FI) / Δ(T) and Δ(δ D FI) / Δ(T)) seems to be stable over time and therefore support the use of water isotopes for temperature reconstructions in Switzerland. • Determining paleotemperatures from stalagmites at different geographical altitudes. • Local temperature – altitude gradient remains rather stable over time. • The temperature sensitivity of δ 18 O FI and δ D FI are stable for Switzerland. [ABSTRACT FROM AUTHOR]

The identification of groundwater recharge sources and flow paths is key for sustainablegroundwater management. Mixing ratios from different water sources are commonlyestimated using end-member mixing analysis (EMMA). Although the importance of the correct identification of the end-members and theinfluence of measurement uncertainty have been recognized in the literature, so far theseuncertainties have only been partly represented in the model structure. EMMA assumes thatconservative tracer concentrations of a water sample are represented solely by a mixtureof samples from predefined end-members. However, even a careful selection ofend-members (e.g. with PCA) cannot exclude the possibility of a contribution from anunknown, unmeasured end-member. Whenever such an additional end-member existsthe standard model assumptions are violated and the resulting mixing ratios arebiased. We present a Bayesian extension of the EMMA with two novel features to deal with theaforementioned problems. For the estimation of the mixing ratios the presented modelconsiders explicitly i) the measurement uncertainty of the tracer concentrations and ii) thepossibility of an unknown residual end-member. Unlike the classical EMMA, this allows forextensive uncertainty quantification by providing the posterior distribution for all quantities.Not only the uncertainties of end-member mixing ratios can be expressed, but also themixing ratio of the residual end-member and its tracer concentrations are quantified.Additionally, tracers with large measurement errors can be identified. Such extrainformation is valuable to design the experimental setup of future measurements. Themodel is also flexible regarding assumptions of the measurement uncertainties. Forexample absolute and relative errors or non-Gaussian distributions can be implementedreadily. We applied the model at a major water supply system in Switzerland to estimate mixingratios between artificially infiltrated surface water and regional groundwater at severaldrinking water and observation wells. We used a set of tracers including 4helium (He)analyzed on-site with a newly developed portable membrane-inlet mass spectrometry system.As an inert noble gas He serves as a very conservative tracer. The two selected end-memberscontribute together to at least 90% for 17 wells. For five wells, however, the model identifieda substantial contribution (up to 45%) of an unknown residual end-member. Thisresult can be used to confirm or revise the conceptual process understanding. Theexplicit consideration of measurement uncertainty enabled us also to quantify thebenefits of using on-site 4He measurements as a new tracer for estimating mixingratios. We belief that explicitly expressing assumptions about model structure deficits andmeasurement uncertainties has two main advantages: it protects from over-interpreting noisydata and it encourages the scientific community to discuss, challenge, and test differentunderlying model assumptions. [ABSTRACT FROM AUTHOR]

The oxygen dynamics in the hyporheic zone of a peri-alpine river (Thur, Switzerland), were studied through recording and analyzing the concentration time-series of dissolved argon, oxygen, carbon dioxide, and temperature during low flow conditions, for a period of one week. The argon concentration time-series was used to investigate the physical gas dynamics in the hyporheic zone. Differences in the transport behavior of heat and gas were determined by comparing the diel temperature evolution of groundwater to the measured concentration of dissolved argon. These differences were most likely caused by vertical heat transport which influenced the local groundwater temperature. The argon concentration time-series were also used to estimate travel times by cross correlating argon concentrations in the groundwater with argon concentrations in the river. The information gained from quantifying the physical gas transport was used to estimate the oxygen turnover in groundwater after water recharge. The resulting oxygen turnover showed strong diel variations, which correlated with the water temperature during groundwater recharge. Hence, the variation in the consumption rate was most likely caused by the temperature dependence of microbial activity.